Question

In my lab manual this is written but I can’t see how this is true

The heat of neutralization of weak acids by strong bases is less than 13.7 Cal/mole because the weak acid becomes completely ionized before the neutralization reaction by absorption of heat.

If the weak acid can absorb heat before the neutralization reaction to completely ionize, then why do we say that it doesn’t fully ionize in the first place? Before reading that text I thought that the reason was that less than 1 mole of H+ ions would be in the solution so less water will form therefore less heat will be released from the neutralization reaction, and that the difference in the heat released is the amount of heat that is needed to fully ionize the remaining weak acid. Where have I gone wrong?

Answer 1

The equilibrium between the reactants and products in weak acid ionization results in only a fraction of the acid molecules dissociating into ions, while the rest remain intact. However, during the neutralization reaction with a strong base, the weak acid reacts completely to form water and a salt. Therefore, the heat of neutralization for weak acids is less than strong acids due to the complete ionization during the neutralization reaction.

Step-by-step explanation:

The reason why weak acids do not fully ionize in water is due to their equilibrium reactions. In an equilibrium reaction, there is a balance between the reactants and products.

When a weak acid is dissolved in water, only a fraction of the acid molecules will dissociate into ions, while the remaining acid molecules remain intact. This is because the equilibrium favors the formation of the weak acid rather than its dissociation.

However, during the neutralization reaction with a strong base, the weak acid reacts completely with the base to form water and a salt. This means that all the weak acid molecules in the solution will participate in the reaction, resulting in complete ionization of the acid. The heat that is absorbed during this ionization process contributes to the lower heat of neutralization for weak acids compared to strong acids.

So, the statement in your lab manual is referring to the heat of neutralization specifically, not the overall ionization of weak acids in water.